That is a very reasonable question. Two parts to the answer.
I sometimes cut up fairly big wood, and I need really flat and true surfaces to be able to read the wood grain well, and to not waste any wood. For some work, if I have to trim even an inch off a block to clean it up it can make the final turned piece much smaller.
Second part is that I admit I've never cut where a 42 inch bar would not reach halfway, I've had it only reach halfway by a couple of inches. But then I end up with a surface that is not perfectly flat, and lose valuable size in the pieces I can get from the wood.
So in truth I can get by without a bar longer than 42 inches, but I would sometimes make life simpler and better. And I am not getting younger, so seems like if I'm going to get one I should just do it. (The mid life crisis sounding part of it all.)
Here's a pic I've posted before of a piece that is only 53 inches where I am cutting it, but a longer bar than my 42 would have been handy, especially on site where they want me to cut fast and get out of there. Also sometimes if a big stick is being craned, then it is easier and quicker to be able to cut off a big piece from one side of the log in one pass.
http://www.billluce.com/pumkinjpgs/pumkinjpgs/8557.JPG
Sometimes the wood I am blocking is very expensive and valuable, like certain fancy big leaf maple or large madrone burls. So it's criticl that I maximize my yield from the wood. For example, it's easy for 3or 4 big madrone burls to cost me over $5000, besides the fact I have to drive almost a thousand miles to bring them home. So dead accurate cutting is critical. Some of that slabbing is done with one of my alaska mills, and there have been times when I need a longer bar than a 42 incher when slabbing these burls.
Sounds like you need a Lucas Mill with the slabing attachment. And after you buy one can I borrow it? Thanks in advance.
I do wish Stihl only had one size mount but I suppose they had a good reason to bump the size up.